The present invention relates to frequency selective electrical filters and more particularly to diplexers for splitting an input signal into spectrum components having complimentary frequency spectra and to multiplexers comprising multiple interconnected diplexers.
Diplexers comprising filters connected between first and second quadrature hybrid couplers configured as a splitter and a combiner have been commonly employed to accomplish the functions of combining and/or separating of signals having different frequencies. Such conventional hybrid-filter diplexers employ substantially identical filters to divide the bandwidth of the diplexer into two frequency bands such that input signals having frequencies within one band are reflected back by the filters into the first hybrid while input signals having frequencies is within the other band are passed by the filters to the second hybrid. Input signals are thereby divided between a port of the first hybrid serving as a first diplexer output port and a port of the second hybrid serving as a second diplexer output port in accordance with frequency.
However, such hybrid-filter diplexers suffer from a serious limitation in that the bandwidth of the diplexer is limited to the bandwidth of the quadrature hybrids which are inherently narrowband and cannot provide acceptable equal power division of input signals between hybrid terminals over much more than a single octave in frequency range. Unfortunately, quadrature hybrids are key components of diplexers for which substitutes having improved broadband performance are not readily available and, consequently, the narrowband performance of diplexers has proved to be an intractable problem.
It is an object of the present invention to provide a new type of diplexer which is broadband providing acceptable performance over an extended frequency range approaching a full decade in breadth.
It is another object of the present invention to provide a broadband diplexer utilizing readily available and reasonable cost components and in the interests of simplicity and economy employing identical filters coupled between its splitter and combiner components.
It is a further object of the present invention to provide a diplexer having infinite isolation between its input and output ports and having consistent and equal impedance between its input and output ports over the entire frequency spectrum (providing all ports are appropriately terminated in matching impedances) which allows for the cascading of diplexers in an multiplexer networks.